We can think of a sound signal either as a function of time or as a function of frequency. The two ways are related by what is called a Fourier transform, so they are often written as $f(t)$ and as $\tilde f(\omega)$, using the same letter, but with the "tilde" above one of them indicating that one is the Fourier transform of the other. From these functions it is also possible to construct other functions of both time and frequency, such as the Wigner function.
I would take "pitch" as more-or-less another name for frequency. Audacity help says "Generally synonymous with the fundamental frequency of a note, but in music, often also taken to imply a perceived measurement that can be affected by overtones above the fundamental."
The squared amplitude of a function is the square of the function, properly speaking the norm, written as $|f(t)|^2$ and $|\tilde f(\omega)|^2$. This function of time or of frequency is always positive or zero, whereas the original function can be positive or negative (or it can be a 2-valued function, which in the time coordinates represent both the signal and its rate of change at a given time). It's common to call the "squared amplitude" the amplitude, just to confuse matters.
Sound level sometimes refers to the logarithm of the squared amplitude, as Georg says, but sometimes not. Wikipedia, for example, suggests that a loudness meter reports on a logarithmic scale, whereas sound level might mean either linear or logarithmic scale. One test for what is meant is whether the signal level is followed by "dB", which will always indicate a logarithmic scale.
Ultimately I'd say that it's the mathematical relationships between these objects that one tries to understand, but a lot can be understood just by experimenting with something like Audacity (which I think is simpler to use than the software mentioned by @whoplisp, although also less powerful), using the "Analyze->Plot Spectrum" option for various sounds. Here's a plot for me whistling, apparently at about 1300Hz (the fundamental frequency) in the 0.026 seconds that I analyzed here, but with various higher frequencies also represented.
When reading something about sound or music, there are various technical usages that are not very standardized, and one often has to figure out what's meant by the overall context. There are many different expert groups that have a strong interest in signal analysis of various kinds, and they have often developed specialized languages appropriate to their own interests.